1. Field of the Invention
The present invention relates to a light source unit, an optical scanning device, and an image forming apparatus, and more particularly to a light source unit for emitting plural light beams, an optical scanning device including the light source unit, and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
A Vertical Cavity Surface Emitting Laser (hereinafter, also referred to as a “VCSEL”) is for outputting light in a vertical direction to a surface of a substrate. The VCSEL is advantageous over a semiconductor laser element of an end face light-emitting type that emits light in a direction parallel to the substrate surface, because the VCSEL is low cost and high performance and arrays can be easily formed with VCSEL.
Furthermore, there are high expectations for using VCSEL as a light source for optical communications such as optical interconnections, a light source for optical pickup, and a light source for an image forming apparatus such as a laser printer. Therefore, research and development relevant to VCSEL are actively implemented, and some of the results of R & D are put into practical use.
Generally, in an optical system using VCSEL as a light source, light emitted from the light source is reflected by lenses and glass in the optical system, and may return to the light source. Because of the light that has returned to the light source (returning light), the laser oscillation in the VCSEL becomes unstable, and the amount of light emitted from the light source varies in an irregular manner over time. The amount of light may vary in nano-second order or micro-second order.
The stability in the amount of light emitted from the light source is an important property when the VCSEL is applied to image forming and communications.
Depending on the image forming apparatus, even a slight percent of variation in the amount of light may lead to serious problems. Thus, high-quality images cannot be formed with a light source that emits unstable amounts of light. In communication applications, if the light source emits unstable amounts of light, signal transmission properties become degraded and optical communications cannot be performed with good performance.
Therefore, various methods have been proposed, such as a method of increasing resistance to returning light (see, for example, patent document 1), or a method of reducing returning light (see, for example, patent documents 2 and 3).
Patent document 1 discloses a surface-emitting laser element in which a resonator is formed by a lower multilayer film reflection mirror and an upper multilayer film reflection mirror. An active layer is placed between the lower multilayer film reflection mirror and the upper multilayer film reflection mirror. The relaxation oscillation frequency at a bias point in the resonator is set so as to exceed the optical communications frequency that modulates laser beams output from the surface-emitting laser element.
Patent document 2 discloses a surface-emitting semiconductor laser including a semiconductor substrate, an active layer provided above the semiconductor substrate, an emitting surface provided above the active layer for emitting laser beams generated at the active layer in a direction vertical to the semiconductor substrate, and an absorption layer provided above the emitting surface for absorbing part of the laser beams.
Patent document 3 discloses a surface-emitting laser module having at least a surface-emitting laser chip and a monitoring photo-detector mounted on a TO header. The surface-emitting laser module has a cap with a window coated by a film having a predetermined transmittance (less than or equal to 40%).
However, when a surface-emitting laser array and an apertured plate are used in combination, the methods disclosed in patent documents 1 through 3 may have the following problem. Specifically, the amount of light that is emitted from the surface-emitting laser array and then passed through the apertured plate may not be stabilized.
Inventors of the present invention made various studies regarding a device including a surface-emitting laser array having plural light emitting units as a light source, and an optical system including an apertured plate having an aperture for adjusting the light beams emitted from the light source. Accordingly, it was found that the variation in the amount of light passing through the aperture is not only affected by light that is emitted from light emitting units and then returns to the same light emitting units as known conventionally. It was found that the variation in the amount of light is also affected by light that returns from other light emitting units.
The variation in the amount of light passing through the aperture, which is caused by light returning from other light emitting units, occurs only when the surface-emitting laser array and the apertured plate are used in combination. Therefore, this factor was not much noticed in the past.
A configuration including a combination of an array of light emitting units which is a feature of VCSEL, and an apertured plate which is a basic element in optical systems, is applied to various fields that are targets of research in recent years and continuing.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-252032    Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-86027    Patent Document 3: Japanese Laid-Open Patent Publication No. 2007-103576